(1) Field of the Invention
The present invention relates to a method for the correction of a defective pixel in a liquid crystal display device, and in particular, to a method for the correction of a defect in a liquid crystal display device, wherein a defective pixel is corrected by irradiation with laser beams.
(2) Description of the Prior Art
Liquid crystal display devices can be driven at low voltages and low electric power, and have been utilized and commercially available as small, thin-type flat panel displays in a wide range of products. Matrix-type liquid crystal display devices are known as one type of such liquid crystal display devices. In particular, a liquid crystal display device with an active matrix driving system is configured such that switching elements such as TFTs (Thin Film Transistors) or MIM (Metal Insulator Metal) Diodes, or the like, are connected to pixel electrodes which are arranged in a matrix-like pattern on one of a pair of glass substrates which are bonded together. Selection and non-selection of all the pixel electrodes is carried out by the switching operation of these switching elements, to achieve display operation. For this reason, a liquid crystal display device of an active matrix driving type does not exhibit crosstalk at the time of non-selection, therefore achieving high quality display. Such crosstalk is a drawback of liquid crystal display devices of a simple matrix driving type.
Since the aforementioned switching element generally has a multi-layered structure consisting of semiconductor layer(s), insulation layer(s), and various types of electrodes for driving of the liquid crystal, which are layered onto a glass substrate, patterning of the layers onto the glass substrate is repeated in order to manufacture the switching element. For this reason, there are cases in which defective switching elements which do not function normally due to disconnection, short-circuit, or the like, are produced at the time of manufacture of the switching elements. Thus, pixels comprising pixel electrodes connected to defective switching elements become defective pixels to which voltage can not be applied. Particularly in the case of liquid crystal display devices in Normally White Mode, which is a display operation mode wherein the display screen is white when voltage is not applied, these defective pixels are found as bright-spot defects through which light is constantly transmitted. Besides a defect in the switching element, damage to a pixel electrode or aligning film can also cause a bright spot.
Various methods for correcting bright-spot defects by using laser beams to irradiate the area around the bright-spot defect in the liquid crystal display panel have been proposed as methods for the correction of bright-spot defects in liquid crystal display devices. For instance, in the case of "A Liquid Crystal Display Device" of Japanese Patent Application Laid-Open Hei 1 No. 187,532, a method is disclosed which reduces the amount of light transmitted through a defective pixel by using laser beams to damage a polarizing plate in the area of the defective pixel. In the case of "A Method for the Manufacture of an Active Liquid Crystal Panel" of Japanese Patent Application Laid-Open Hei 3 No. 21,928, a method is disclosed for reducing the amount of light transmitted through a defective pixel by means of burning the color filter of the defective pixel with laser beams so that it becomes gray, and heating the rubbing surface on the side of the pixel electrode so that the orientation of liquid crystal is disturbed. Furthermore, in the case of "A Liquid Crystal Display Device and A Method for the Correction of A Defect in A Liquid Crystal Display Device" of Japanese Patent Application Laid-Open Hei 4 No. 301,615, there is disclosed a method for reducing the amount of light transmitted through a defective pixel by means of using laser beams to form an indentation on the glass substrate corresponding to the defective pixel, and roughening its bottom surface. In the case of "A Method for the Correction of A Defect in A Liquid Crystal Display Device" of Japanese Patent Application Laid-Open Sho 60 No. 243,635, there is disclosed a method for reducing the amount of light transmitted through a defective pixel by using laser beams to irradiate the area of the defective pixel in order to burn aligning film and pixel electrode material so that they lose their capability to control the orientation of the liquid crystal layer, whereby the orientation of the liquid crystal molecules in this area becomes random.
However, in the case of each of the aforementioned methods for the correction of a bright-spot defect in a liquid crystal display device, the amount of light transmitted through the defective pixel is merely reduced to an intermediate level (an intermediate tone) between that characteristic of a voltage-activated state and voltage nonactivated state; the light transmitted through the defective pixel is not completely blocked.
For this reason, when a liquid crystal display device wherein a defective pixel has been corrected to an intermediate tone as above is used in a magnifying projection-type liquid crystal projection apparatus, all of the pixels are magnified so that the defective pixel which has been corrected is also magnified. Thus, in such cases, the problem occurs that the defective pixel is conspicuous and causes a decline in the quality of the projected image when the amount of light transmitted through the defective pixel is merely reduced to an intermediate tone.
Furthermore, none of the aforementioned methods for correction of a bright-spot pixel clearly mention the amount of energy of the laser beam applied to the defective pixel. When a laser beam is used to correct a defective pixel, it is normally necessary to irradiate substantially the entire surface of the defective pixel with the laser beam. Due to this, if the defective pixel is irradiated with laser beams at an energy level equal to or higher than a prescribed value, the problem occurs that even the adjacent, normal pixels are affected, resulting in degradation of the display quality of the liquid crystal display device after correction.
Specifically, as shown in FIG. 1, when a laser beam is used to correct a bright-spot pixel, an irradiation spot 19a, whose shape corresponds to that of a bright spot pixel 18 in a correction area 6a, and a laser irradiation-affected area 20a are generally formed. In this case, since the irradiation spot 19a is made large enough to correct the bright-spot pixel 18, the laser irradiation-affected area 20a also becomes large, such that pixels 17 adjacent to the bright-spot pixel 18 are affected. Thus, when the bright-spot pixel 18 is corrected, the adjacent normal pixels 17 are affected by the laser beam, resulting in deterioration of the display quality of the liquid crystal display device after correction.